Process for producing sintered articles



Nov. 24, 1964 L. VON BOGDANDY ETAL PROCESS FOR PRODUCING SINTERED ARTICLES F'iied Oct. 19, 1961 WIG VON BOGDA N NIKOLA STRAN Jn venfor's:

AGENT United States Patent ,Ofifice 3,158,472 Patented Nov. '24, 1 964 This invention relates to sintered bodies and to a process for producing such bodies from comminuted metal, particularly iron.

One of the difficulties encountered in the manufacture of sintered machine parts and other articles is the shrinkage that takes place during sintering. One method of overcoming this drawback in the case of iron involves the use of additives such as copper, zinc or aluminum which dissolve into the iron and lead to an increase in particle volume compensating for the shrinkage. Since the additives are generally more expensive than the basic powder material, this answer to the problem is not economically practical. Also in many cases the addition of a foreign material is not admissible because of its effect on the characteristics of the finished article.

It is an object of this invention to provide a simple and efficient process for so treating metal powder as to minimize shrinkage during subsequent sintering.

It is a more particular object of the invention to provide a process for reducing the shrinkage of iron powder without the admixture of other metals.

It is also an object of this invention to provide a metal powder of any desired degree of purity adapted to be used in the formation of more or less porous sintered bodies that are substantially true to the dimensions of a mold in which they are formed.

It has been found, in accordance with the instant invention, that the shrinkage of comminuted metal during sintering can be greatly reduced by initially providing the metal as a mixture of a coarse and a fine fraction, preferably present as a major and a minor proportion, respectively, and subjecting either or both of these fractions to a selective internal deformation prior to sintering whereby the fine particles will be more severely stressed than the coarser particles. It is believed that this results in an expansion of the smaller particles relative to the larger particles during sintering, this relative expansion counteracting the normally unavoidable shrinkage.

A more specific feature of this invention resides in the use of a mixture consisting essentially of two grades of iron powder, i.e., a fine and a course fraction. At least 70% by weight of the fine-grain powder has particle diameters ranging between and 0.06 mm. while 90% of this powder has particle diameters less than 0.1 mm.; in the coarse-grain powder 70% of the particles have a diameter greater than 0.1 mm. and 80% of this powder has particles larger than 0.06 mm. in diameter. These two powder fractions are mixed, pressed into a blank and sintered in the usual manner.

The fine-grain component generally constitutes 5 to 40% by weight of the entire mixture while the coarsegrain component accounts for 60 to 94%. The balance, if any, may conventional additives designed to modify the properties of the final product, e.g., to lower its coeflicient of friction for bearing races and the like.

The selective stressing of the fractions according to our invention can be achieved by various methods but an effective manner of accomplishing this end is subjecting the fine powder to compression and then recomminuting it to its original particle size. In contradistinction thereto, the coarse powder is stress-relieved by annealing it at a temperature of at least 500 C., but substantially below the sintering level, before mixing it with the fine-grain component. The powders are mixed in the proportions specified above, compressed, and sintered. The precompression of the fine fraction and the annealing of the coarse fraction may be employed alternatively or together.

Generally, the two grades of powder can be produced I by various processes. It is convenient, however, to use commercially available sinter iron powder and to separate it into coarse and fine fractions by sifting it. Any remaining intermediate-size fraction can either be used for other purposes or be ground up into fines.

It has been found, surprisingly enough, that the use of a comminuted mixture according to this invention results in an unexpected great'reduction in shrinkage during the sintering process. The shrinkage of a blank made from this mixture of powder is considerably less than that of one made from conventional powder or from either of the aforedescribed components alone, and is further significantly reduced by the pretreatment of the fractions.

Example An iron powder produced by the reduction of ore concentrates exhibited upon analysis the following impurities: 0.08% C., 0.5% O, 0.025% P, 0.02% S, and 0.8% SiO (percentages by weight). The grains in the coarse fraction of the powder had diameters between 0.2 and 0.3 mm. while the fine grains ranged between 0.042 and 0.06 mm. in diameter. The coarse powder was heated to 600 C. and the fine powder was deformed by subjecting it to a pressure of 8 metric tons per sq. cm. Both fractions of the powder were then reduced to their original grain size and mixed.

A test specimen was compressed in a mold with a force of 3 metric tons per sq. cm. and sintered at 1,150 C. The shrinkage was determined by measuring the linear dimensions of the specimen before and after sintering. The low compression of 3 tons per sq. cm. was chosen in order to yield relatively high absolute values for the shrinkage. The results obtained with different proportions of the two fractions are shown in the following table:

Sample Composition by weight Percent Shrinkage 100% coarse powder 0.75 coarse, 15% fine powder. 0. 5 50% coarse, 50% fine powder- 1. 5

In the accompanying drawing, FIG. 11 illustrates a bearing race of sintered iron made by the process according to this invention, FIG. 2 showing an enlarged section thereof. The enlarged section A (FIG. 2) shows that the sintered powder is composed of particles of two different sizes, namely fine grains 1 with diameters between 0.04 and 0.06 mm. and coarse grains 2 with diameters between 0.2 and 0.3 mm. The fines 1 will be seen to occupy the spaces between the larger particles 2 so as to result in a sintered body of substantially uniform density throughout.

I claim:

1. A process for producing a sintered article from a coarse-grain and a fine-grain fraction of iron powder, the bulk of said coarse-grain fraction consisting of particles greater than 0.1 mm. in diameter, the bulk of said fine-grain fraction consisting of particles less than 0.1 mm. in diameter, comprising the steps of increasing the--internal stresses of the particles of said fine-grain fraction relatively to those of the particles of said coarsegrain fraction by mechanically compacting the particles of said fine-grain fraction, recomminuting said fine-grain t a o o sub tantially t or g na ;r c ze, m xin I armaior propo ti n of sa oa s -g fraction w a minor proportion of said fine-grain fraction, said minor fraction constituting between substantially 5% and 40% by weigh .Of the mixture, molding the mixture into a blank 0f an article 'to'be produced, and sintering said -1; ;lan

2. A process according to claim 1 wherein said fine- 1 grain fraction consists in a proportion upwards of sub- 3. A process according to claim 1 wherein the step of relatively increasing said internal stresses further includes annealing the particles of said coarse-grain fraction at a temperature substantially below the sintering level.

4. A process according to claim 3 wherein the particles of said fine-grain fraction are compacted under a pressure of the order of eight tons per square centimeter, further comprising the step of annealing the particles of said coarse-grain fraction ata temperature of the order of 600 C. 7

5. A process according to'claim 1 wherein said coarsegrain and fine-grain fractions are initially obtained from a mixture of different particle sizes of iron powder.

References Cited in the file of this patent UNITED STATES PATENTS 2,179,960 Schwarzkopf Nov. 14, 1939 2,229,330 Langharmner et al. Jan. 21, 1941 2,306,665 Schwarzkopf Dec. 29, 1942 2,857,270 Brundin Oct. 21, 195.8 

1. A PROCESS FOR PRODUCING A SINTERED ARTICLE FROM A COARSE-GRAIN AND A FINE-GRAIN FRACTION OF IRON POWDER, THE BULK OF SAID COARSE-GRAIN FRACTION CONSISTING OF PARTICLES GREATER THAN 0.1 MM. IN DIAMETER, THE BULK OF SAID FINE-GRAIN FRACTION CONSISTING OF PARTICLES LESS THAN 0.1 MM. IN DIAMETER, COMPRISING THE STEPS F INCREASING THE INTERNAL STRESSES OF THE PARTICLES OF SAID FINE-GRAIN FRACTION RELATIVELY TO THOSE OF THE PARTICLES OF SAID COARSEGRAIN FRACTION BY MECHANICALLY COMPACTING THE PARTICLES OF SAID FINE-GRAIN FRACTION, RECOMMINUTING SAID FINE-GRAIN FRACTION TO SUBSTANTIALLY ITS ORIGINAL PARTICLE SIZE, MIXING A MAJOR PROPORTION OF SAID COARSE-GRAIN FRACTION WITH A MINOR PROPORTION OF SAID FINE-GRAIN FRACTION, SAID MINOR FRACTION CONSTITUTING BETWEEN SUBSTANTIALLY 5% AND 40% BY WEIGHT OF THE MIXTURE, MOLDING THE MIXTURE INTO A BLANK OF AN ARTICLE TO BE PRODUCED, AND SINTERING SAID BLANK. 